The invention relates to the field of wireless communications and, more particularly, to subscriber handover between cellular communications networks.
In a cellular communications network, a subscriber call may be handed over among various cellular communications nodes during the course of the call. As the subscriber travels between coverage areas of the individual communications nodes within a communications network, a cellular switching system determines whether a different communications node within the network can better serve the subscriber. When the cellular switching system determines that a different communications node within the network can provide a higher quality channel, the call is handed over in order to maintain the highest quality communications channel. This allows the subscriber to move freely within a particular network as a call is handed over from one communications node to another.
When a low earth orbit satellite cellular communications network is providing services to a subscriber, a call is handed over between adjacent satellite communications nodes in a manner similar to that performed by a terrestrial cellular communications network. In this type of cellular communications network, calls are handed over between adjacent satellite communications nodes due to the satellite""s motion relative to an earth-based subscriber.
When a subscriber within a terrestrial cellular communications network moves outside of the coverage area served by the network, the voice and signal quality degrade as the user moves farther away from the outermost terrestrial cellular communications node. As the subscriber moves completely out of the coverage area, the communications network will eventually drop the call. At this point, the subscriber can neither send nor receive information through the communications network.
A partial solution to this problem can be realized through the use of a dual mode communications unit. Through the use of a dual mode communications unit, the subscriber can subsequently contact an appropriate satellite cellular communications node and initiate a new telephone call through the satellite communications network. However, the subscriber is required to redial the previously called party, or wait for an incoming call from the called party to be routed through the satellite communications network to the particular subscriber communications unit. In either case, the subscriber must first successfully register and update their location with the satellite network.
A similar but less problematic situation arises when a subscriber equipped with a dual mode communications unit moves from an area covered by a satellite communications network to an area additionally covered by a terrestrial cellular communications network while engaged in a telephone call. In this case, the subscriber can be making use of relatively expensive satellite resources in order to conduct the telephone call while relatively inexpensive terrestrial resources could be used to conduct the call. In order to take advantage of the less expensive terrestrial communications resources, the subscriber is required to terminate the call through the satellite communications network and re-establish the call through the terrestrial communications network. This represents an undesirable situation where the subscriber""s workload is increased in order to take full advantage of inexpensive terrestrial cellular communications.
Therefore, it is highly desirable for the subscriber communication unit to seamlessly roam between a terrestrial and satellite communications network during an active call. Such a system would allow a subscriber leaving an area covered by a terrestrial communications network to seamlessly connect with a global satellite communications network. In a similar manner, such a system would also allow a subscriber telephone call to be handed over from a satellite cellular communications network to a terrestrial communications network when the subscriber travels to a location within the terrestrial communications network""s coverage area. This would ensure that the communication unit always makes use of the lowest cost resources available.